63 Technical Description Precision-machining robot system Preface Thanks to their versatile configurations, affordable price, and advanced ability for faithful repetition of operation instructions, industrial robots have evolved through application for automation of welding, painting, and other operations centering on the automobile industry. Furthermore, an increasing number of users have in recent years been making use of offline teaching with computers to shorten the time required for production planning proposals and line startup. To better meet these needs, industrial robots have acquired greater absolute accuracy for precise positioning into the specified spatial position as well as repeatability for faithfully repeating operations. As robots have increased in precision through technological advances, demands for robots as substitutes for NC machine tools have also increased. But since robots generally have a cantilevered structure resulting in low rigidity, they are not suited for micron-order ultra-precision machining. Nevertheless, because robots offer a wide operations range and reasonable price levels, their application is expected in areas where micron-order precision is not necessary and the use of high-priced large machine tools would constitute a waste of resources. In addition, even in laser machining and other machining processes requiring sub-millimeter precision, where robot applications have previously been difficult due to insufficient precision, the possibility is increasing for application of low-priced industrial robots. 1 Application of robots to machining processes In the process of developing new automobile models, as many as 40 to 80 seat prototypes processed out of urethane or Styrofoam rolls, etc., are produced for each model by the completion of the final design. While NC machine tools have been used in the past for this kind of prototype machining, the use of high-priced NC machine tools constitutes an overperformance in terms of precision. Moreover, sublimation patterns of prototype molds for industrial machinery, camera or printer models, and wood machining require the same level of precision as the seat molds or less. If industrial robots are used in these fields, we can expect the following benefits. ① Cost reductions due to use of low-price robots. ② Achievement of wide-ranging 5-axis machining, including wraparound operations. ③ Combined use of traveling equipment and turntables for selecting flexible system configurations in accordance with workpiece size. However, achieving substitution of NC machine tools will require an industrial robot with the necessary precision, and use of 3D CAD/CAM data to perform machining while using simulations to check for robot interference, etc. In fact, there are areas where robot substitution is possible, and areas where only NC machine tools can be applied. These areas are shown together with their Technical Description The enhanced precision of industrial robots and progress in their peripheral technologies have allowed robots to enter the field of machining. The wide operation range and affordability of the system offer the prospect of replacing NC machine tools in the prototype modeling field. This paper provides a commentary on milling robot systems for precision machine and presents examples of precision processing with robots.
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Precision-machining robot system · manipulating the machining speed or natural frequencies. Therefore, we use a method of adding signals to the motor torque command for extinguishing
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63
Technical Description
Precision-machining robot system
Preface
Thanks to their versatile configurations, affordable price,
and advanced ability for faithful repetition of operation
instructions, industrial robots have evolved through
application for automation of welding, painting, and other
operations centering on the automobile industry.
Furthermore, an increasing number of users have in recent
years been making use of offline teaching with computers
to shorten the time required for production planning
proposals and line startup. To better meet these needs,
industrial robots have acquired greater absolute accuracy
for precise positioning into the specified spatial position as
well as repeatability for faithfully repeating operations.
As robots have increased in precision through
technological advances, demands for robots as substitutes
for NC machine tools have also increased. But since robots
generally have a cantilevered structure resulting in low
rigidity, they are not suited for micron-order ultra-precision
machining. Nevertheless, because robots offer a wide
operations range and reasonable price levels, their
application is expected in areas where micron-order
precision is not necessary and the use of high-priced large
machine tools would constitute a waste of resources. In
addition, even in laser machining and other machining
processes requiring sub-millimeter precision, where robot
applications have previously been difficult due to
insufficient precision, the possibility is increasing for
application of low-priced industrial robots.
1 Application of robots to machining processes
In the process of developing new automobile models, as
many as 40 to 80 seat prototypes processed out of
urethane or Styrofoam rolls, etc., are produced for each
model by the completion of the final design. While NC
machine tools have been used in the past for this kind of
prototype machining, the use of high-priced NC machine
tools constitutes an overperformance in terms of precision.
Moreover, sublimation patterns of prototype molds for
industrial machinery, camera or printer models, and wood
machining require the same level of precision as the seat
molds or less.
If industrial robots are used in these fields, we can
expect the following benefits.
① Cost reductions due to use of low-price robots.
② Achievement of wide-ranging 5-axis machining,
including wraparound operations.
③ Combined use of traveling equipment and turntables
for selecting flexible system configurations in
accordance with workpiece size.
However, achieving substitution of NC machine tools
will require an industrial robot with the necessary precision,
and use of 3D CAD/CAM data to perform machining while
using simulations to check for robot interference, etc.
In fact, there are areas where robot substitution is
possible, and areas where only NC machine tools can be
applied. These areas are shown together with their
Technical Description
The enhanced precision of industrial robots and progress in their peripheral technologies have allowed robots to enter the field of machining. The wide operation range and affordability of the system offer the prospect of replacing NC machine tools in the prototype modeling field. This paper provides a commentary on milling robot systems for precision machine and presents examples of precision processing with robots.
64Kawasaki Technical Review No.172 December 2012
relationship to machining precision and market scale in
Fig. 1. The machining precision required depends on the
target workpiece. Therefore, we created a milling robot for
practical application by increasing the robot precision and
realizing a system demanded by the market.
2 Technology for achieving precision machining
To use industrial robots, whose development has mainly
centered on the teaching playback function due to
repetitive operations, as substitutes for NC machine tools,
the following issues need to be addressed.
① Improvement of absolute accuracy through correction
of machine difference and deflection
② Software for conversion of multipoint (hundreds of
thousands of points) machining data into robot
programs
③ Suppression of micro-vibrations in robots that occur in
resonance with periodic variations (ripples) generated in
reducers
④ Precision tool measurement with no machining position
offset due to changes in the end mill posture
(1) Improvement of absolute accuracyWhile industrial robots have a precision of 0.1 mm for
repeatability (precision in the reproduction of teaching
positions), their absolute accuracy (precision in moving to
positions specified with coordinate values) is not as high.
This is due to robot machining and assembly errors, zero-
point errors of the joint angle sensors, or arm deflection. To
ensure accurate positioning, we developed a technology
that takes these error factors into consideration to correct
the command position and hold the robot’s average
absolute accuracy to 0.5 mm or less. We also perform
measurements before shipment to identify the robot part
dimensions, the joint angle sensor zero point, and the
rigidity of each part, and input the data to the robot
controller, to achieve high-precision position correction.
(2) Conversion from G-code to robot programIn general, output from CAD/CAM to NC machine tools is
performed in the industry standard format known as
G-code. We have developed software for automatically
generating robot programs from this G-code machining
data. Since the user can continue using the G-code
machining data generated for NC machine tools, the milling
1.0mm
Machining process: Mass-production metal molds, etc.
Markets where NC machine tools are applied
Machining process: Sublimation pattern casting, etc.